core: [cleanup] split execute method of CreateAllSnapshots

CreateAllSnapshotsFromVmCommand#executeVmCommand method is too long.
it's going to be even longer after adding memory snapshot, so it's a
good point to organize it by spliting the different concerns in this
method to several private methods.

Change-Id: Ie401b3aa60abba497237e02c77d8d3a77cef9a20
Signed-off-by: Arik Hadas <ahadas@redhat.com>